The "best band in the world", Tenacious D are Jack Black (vocals/guitar), a comic actor who has been in several films including Saving Silverman and High Fidelity, and Kyle Gass (guitar/vocals) who is a minor actor, appearing in Saving Silverman also. They played "open mic" nights in friends' cafes and bars before getting a short lived HBO series and eventually a record deal.
Actually, I should point out that Kyle wasn't in Saving Silverman (Evil Woman in the UK) with Jack, but he WAS in Shallow Hal with him. Sorry about that.
Jack Black has also been in Jackal with Bruce Willis, and Enemy of the State. He was also the lead role in Shallow Hal. Kyle is in most things that Jack is in, usually as a very small part. Most recently he was Phoebe's mugger friend in one episode of series 9 of Friends. They did some stuff in Foo Fighters videos, particularly 'Learn to Fly', and after becoming mates with the genius that is Dave Grohl, he does a bit of guest drumming for them on their album.